Field in the Industry
The present invention concerns a catalyst for methanation or formation of methane by reaction of hydrogen with carbon dioxide, a mixture of carbon monoxide and dioxide, or a mixed gas containing them as the main components. The invention also concerns method of preparing the catalyst and the process for the methanation using the catalyst.
State of the Art
Global warming due to carbon emissions as a result of combustion of fossil fuels is getting serious, and ways for decreasing the emission have been sought. As one of the countermeasures, the method of producing methane by reaction of carbon dioxide with hydrogen is expected to prevent global warming and supply fuel. Also, hope is placed on the technology of producing fuel gas with high combustion energy by methanation from low combustion energy gas mixture of hydrogen, carbon monoxide and carbon dioxide formed by gasification of coke, coal, biomass, activated sludge, and so on.
To date Raney nickel and catalysts supported on alumina or silica have been examined as the catalysts for methane production by contact reaction of hydrogen with carbon dioxide, carbon monoxide or their mixture. However, because reaction rates with these catalysts are so slow that the reaction should be carried out under a high pressure.
The inventors discovered the fact that ribbon-shaped amorphous alloys, prepared by rapid quenching of the liquid state, consisting of an iron group element such as Ni and Co, and a valve metal such as Zr, Ti, Nb and Ta are effective catalysts for methane synthesis, and disclosed as Japanese Patent Disclosure Nos. 10-43594, 10-244158 and 10-263400. The selectivity to methane formation achieved by these catalysts is almost 100% and the conversion rate is very fast at ambient pressure. However, the process for producing the catalyst alloys, rapid quenching from the liquid state, is not suitable for mass production, and the applicable systems of ribbon-shaped catalysts are limited.
On the basis of the above-mentioned discovery the inventors developed a method of producing powder-shaped catalyst, mass-production of which is much easier than the ribbon-shaped catalysts, and disclosed as Japanese Patent Disclosure No. 2000-254508. This catalyst is composed of Ni supported on tetragonal ZrO2, and prepared by calcination of tetragonal zirconia-type oxides including tetragonal structure-stabilizing elements (Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Eu, Mg and/or Ca), followed by impregnation of Ni and/or Co and final reduction to form metallic Ni and/or Co.
The catalyst disclosed in Japanese Patent Disclosure No. 2000-254508 includes the following embodiments:
(1) A catalyst for carbon dioxide methanation, which comprises Ni and/or Co supported on tetragonal zirconia-type supporter containing a stabilizing element or elements selected from the group consisting of Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Eu, Mg and/or Ca, wherein the ratio of Zr to the sum of Zr and the stabilizing element(s) is 85 atomic % or more and wherein the ratio of Ni and/or Co to the total metallic elements is 0.05-0.5.
(2) The method of preparing the catalyst for carbon dioxide methanation, which comprising the steps of forming tetragonal zirconia-type supporter by addition of one or more of salts selected from the group consisting of Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Eu, Mg and Ca to aqueous zirconia sol under stirring, drying and calcining, and immersing the tetragonal zirconia-type supporter in a solution of a salt or salts of Ni and Co, drying calcining, and final reduction.
As the result of further research the inventors discovered the fact that, instead of impregnation of Ni and/or Co to tetragonal zirconia-type oxide previously prepared as described in Japanese Patent Disclosure No. 2000-254508, preparation of oxide containing all the components necessary for the catalyst and subsequent reduction will give a catalyst with better performance, and thus, achieved the new invention.